PGA's are polysaccharides, commonly present in plants, relatively heterogeneous from a structural point of view and essentially definable as galacturonic glycans (poly (1.fwdarw.4)-.alpha.-D-galactopyranosiluronic acids, sometimes called pectic acids.
Apart from residues of .alpha.-D-galacturonic acid, very prevalent and present in blocks of 25-30 consecutive units, PGA's contain along their chains some residues of neutral monosaccharides--generally not exceeding about 15%--in particular .beta.-L-rhamnose.
The chains of PGA may also contain branches (for example short arabinogalactan chains) bound (1.fwdarw.4) mainly to the rhamnose residues.
One characteristic of PGA is that it forms gels which are stable in aqueous media in the presence of Ca(II) ions, even in the absence of added sugar. Of equal note are mixed thermoreversible gels prepared from acid aqueous solutions in the absence of Ca(II) ions and sucrose, containing PGA and alginates.
Thanks to the complete biocompatibility of their components, such gels are of particular interest in the fields of foodstuffs, pharmaceuticals and cosmetics.
PGA's also exhibit interesting rheological characteristics in aqueous media. Some PGA-Me-x (x being about 50%) from particular vegetable sources contain acetyl groups (about 2-9% by weight) which may on the one hand attenuate these polymers' gel-forming characteristics but also give them remarkable tensioactive and in particular emulsifying properties (stabilization of oil-in-water emulsions).
PGA-Me-x, widely present in plants, may have substantially identical chains to those of PGA's, while an x % fraction of the residues of galacturonic acid in the chain is in the methyl ester form.
The value of x % depends on the natural source used to extract the biopolymers and normally comes within the range of about 10% to about 75%. Depending on the degree of methylation, expressed as x % Me, the PGA-Me-x's exhibit characteristic gel-forming properties in aqueous media not only in the absence of calcium ions but also for example in the presence of sufficient concentrations of sucrose. It is well known that various PGA-Me-x's are used in the food industry, thanks to these very properties.
Chemical modification methods for pectins are known which are based on demethoxylation to increase solubility of the polymer (U.S. Pat. No. 4,016,351 issued Apr. 5, 1977). In the citation of the chemical possibilities, not all the reactions intervening on the hydroxy and/or dihydroxy groups were considered, but only the carboxyl group was targeted. To date, only the synthesis of pectin partial esters of simple aliphatic alcohols, notably ethanol, has been carried out using PGA-Me-x samples, in particular citrus and sunflower pectic acids, (C. G. Kratchnov et al., Carbohydr. Res., 80, 350 (1980); which refers to Z. I. Kertesz, "The Pectic Substances", Interscience, New York (1951); R. McDonnell et al., Arch. Biochem., 28, 260 (1950); and H. Duel, Ber. Schweiz, Bot. Ges., 53, 219 (1943)). However, the method of synthesis employed (PGA-Me-x in alcoholic mineral acids) leads to severe chain degradation and fails to yield mixed esters because of a concomitant, extensive trans-esterification side-reaction.